Magnetron with choke structure for reducing harmonics in output

ABSTRACT

A magnetron greatly reducing the radiation of at least one of unnecessary harmonics contained in the microwave output of the magnetron by providing a simple choke structure within the output portion of the magnetron.

United States Patent [1 1 Oguro [451 Nov. 19, 1974 MAGNETRON WITHCI-IOKE STRUCTURE FOR REDUCING HARMONICS IN OUTPUT [75] Inventor:

[73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Dec. 13, 1973[21] Appl. No.: 426,705

Tomokatsu Oguro, Mobara, Japan [30] Foreign Application Priority DataDec. 18, 1972 Japan 47-144213 Jan. 16, 1973 Japan 48-7073 Jan. 8, 1973Japan 48-5363 [52] US. Cl 331/86, 313/158, 315/3953,

[51] Int. Cl H0lj 23/32, H03b 9/10 [58] Field of Search 331/86-91;

[56] References Cited UNITED STATES PATENTS 2,680,827 6/1954 Randall eta1 331/86 3,315,121 4/1967 Staats 315/3953 X Primary Examiner-JohnKominski Assistant ExaminerSiegfried H. Grimm Attorney, Agent, orFirm-Craig & Antonelli [5 7] ABSTRACT A magnetron greatly reducing theradiation of at least one of unnecessary harmonics contained in themicrowave output of the magnetron by providing a simple choke structurewithin the output portion of the magnetron.

18 Claims, 9 Drawing Figures PAIENTEL NOV 1 91914 SHEET 2 OF 4 FIG.5

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lllll'll MAGNETRON WITH CHOKE STRUCTURE FOR REDUCING HARMONICS IN OUTPUTThe present invention relates to a magnetron, and more particularly tothe structure of the output portion of a magnetron.

Generally, microwaves produced by a magnetron contain many necessaryharmonics. Conventional magnetrons'are of such a structure thatmicrowaves produced are radiated outside as they are. As a result, whensuch a magnetron is used in a microwave cooking oven, for example, thereis the danger that harmonics leak outside through the gap between thecooking chamber and the door and through the observation window todisturb communications. In order to prevent these dangers variouscountermeasures have been taken, some of which are the employment ofelectromagnetic wave absorbers as the sealing material for the door andreduction of the size of the observation window at thecost of theobservability. However, such expedients give rise to such inconveniencesas complicate the structure and impair the function of the cooking oven.

As a prior art of the magnetron provided with a choke for higherharmonics at the output portion of the magnetron there is known onedisclosed in US. Pat. No. 3,559,094. In this patent, however, since thechoke is provided outside the body of the magnetron, thevoltage-withstanding property thereof deteriorates due to' its exposureto the ambient temperature. In order to prevent this inconvenience itemploys an insulating majunction with the accompanying drawings, inwhich:

terial, which, however, gives rise to another inconvenience that thestructure of the magnetron becomes complicated. In addition, themagnetron of the above US. patent has the further disadvantage that anumber of unnecessary harmonics cannot be choked.

Therefore, an object of the present invention is to provide a magnetronof a simple structure without necessity of increasing its componentparts for preventing the harmonics of the microwave produced by themagnetron from radiating outside by choking them in the magnetron.

Another object of the present invention is to provide a magnetron of asimple structure capable of preventing a number of unnecessary harmonicsof the microwave produced by the magnetron from radiating outside themagnetron.

According to the present invention there is provided a magnetroncomprising a cylindrical anode, a cathode disposed coaxially with thecyindrical anode, a plurality of vanes protruding from the inner face ofthe anode towards the cathode for forming cavity resonators, a cathodestem provided gas-tightly at one opening end of the cylindrical anodefor supporting the cathode and insulating the cathode from the anode, ahollow insulating body having at least one opening provided on the sideof the other opening end of the cylindrical anode in a gas-tightrelation with the cylindrical anode, an output conductor one end ofwhich is connected to one ofthe vanes and the other end of which extendsin the hollow of the hollow insulating body, an output portion composedof at least the hollow insulating body and the output conductor, and achoke structure provided inside the output portion for blocking at leastone of the harmonics contained in the microwave output of the magnetron.

FIG. 1 is a partly cutaway elevational view of an embodiment of themagnetron according to the present invention;

FIG. 2 is an elevational cross-sectional view of an important part of amodification of the embodiment of FIG. 1;

FIG. 3 is a partly cutaway elevational view of another embodiment of thepresent invention;

FIGS. 4 and 5 are elevational cross-sectional views of important partsof modifications of the embodiment of FIG. 3;

FIG. 6 is a partly cutaway elevational view of a further embodiment ofthe magnetron according to the present invention;

FIG. 7 is an elevational cross-sectional view of an important part of amodification of the embodiment of FIG. 6;

FIG. 8 is a partly cutaway elevational view of a still furtherembodiment of the magnetron according to the present invention; and

FIG. 9 is an elevational cross-sectional view of an important part of amodification of the embodiment of FIG. 8.

Referring now to FIG. 1 a magnetron 10 includes a hollow cylindricalanode 11 and a cathode 12 disposed inside and coaxially with the anode11. The cathode I2 is provided at its both ends with circular disks l3and 13a for preventing electrons from diverging in the axial direction.A plurality of vanes 14 protrude from the inner face of the anode 11.The vanes 14 are arranged around the cathode l2 and form cavityresonators with themselves. To the upper and lower opening parts of theanode 11 are connected substantially conical iron magnetic pole pieces15 and 16 gas-tightly. Circular openings 15a and provided at the centersof the magnetic pole pieces 15 and 16 are arranged near the circulardisks l3 and l3a.To the upper face of the magnetic pole piece 15 isadhered the larger opening part of a stepped substantially funnel'shapedmetal tube 17. To the smaller opening part of the metal tube 17 isconnected gas-tightly a-cylindrical insulating body 18 of, for example,ceramic to the upper end of which is adhered the opening part of a metalevacuating tube 19 of a cup shape formed by tipping off in evacuationprocess of the magnetron. The evacuating tube 19 has an effective depthl (=k/4n) of one-fourth of the wavelength )t/n of the nth higherharmonic (n 2, 3, of the microwave (the fundamental wavelength A)radiated by the magnetron 10. The upper part of the evacuating tube 19is capped with a metal protecting cover 20 which also serves as anoutput antenna. The tip 19a of the evacuating tube 19 is, afterevacuating the magnetron therethrough, sealed gas-tightly by beingcompressed against one end 21a of an output conductor 21 made of, forexample, oxygen-free copper by hydraulic rollers, for example. The otherend 21b of the output conductor 21 is connected to one of the vanes 14by, for example, soldering through a circular through-hole 15b formed ata part of the magnetic pole piece 15. To the magnetic pole piece l6 acathode stem 31 having a pair of leads 32 for flowing a heat cur rent isfixed for supporting the cathode l2 and insulating the same from theanode 11.

As is apparent from the structure of the above embodiment, the microwaveproduced in the cavity resonator reaches the evacuating tube 19 throughthe output conductor 21. Since the evacuating tube 19 has only arelatively low series reactance for fundamental microwaves, thefundamental microwaves are radiated outside without any interruption.However, the evacuating tube 19 presents a very high series reactancefor a particular harmonic, so that the harmonic component is very littleradiated outside being choked by the series reactance of the evacuatingtube 19.

For example, if the depth 1 of the evacuating tube 19 is taken to beabout M8, the evacuating tube 19 presents a very high series reactancefor the most intense second harmonic, so that the radiation of thesecond harmonic component is greatly reduced. The most intense secondharmonic is reduced by more than db compared with the conventionalmagnetrons one. It is evident that by varying the effective depth 1 ofthe evacuating tube 19 any desired harmonic can be prevented fromradiating outside. When a metal film 22 is provided on the inner face ofthe insulating body 18 such that it connects with the evacuating tube 19as shown in FIG. 2, the effective depth 1 is measured from the lower end22a of the metal film 22.

In the above embodiments the output portion is provided in the axialdirection of the anode 11. However, it may be that the output portion isprovided laterally to and outside of the anode 11 and the outputconductor 21 is connected to the vane 14 through a throughhole formed inthe wall of the anode 11.

In the embodiment of FIG. 3, in addition to the feature that theeffective depth 1 of the evacuating tube 19 is one-fourth of thewavelength Mn of the nth harmonic, a metal cup 23 is held by the outputconductor 21 on its way in such a manner that the center of the bottomof the cup is penetrated by the conductor for preventing the radiationof the n'th harmonic component (n' 9* n) outside. The metal cup 23 hasthe effective depth 1 equal to one-fourth of the wavelength k/n'(=)\/4n') and the mouth of the cup 23 is in a direction towards thevanes 14.

Since the metal evacuating tube 19 and the metal cup 23 present onlyvery low series reactances for the fundamental wave, the microwaveproduced in the cavity resonator and reaching the metal evacuating tube19 through the metal cup 23 on the output conductor 21 is radiatedoutside without interruption. However, the nth harmonic is choked by thevery high series reactance of the metal evacuating tube 19, and likewisethe n'th harmonic is choked by the very high series reactance of themetal cup 23. Consequently, by selecting the effective depths l and 1 ofthe metal evacuating tube 19 and the metal cup 23 to be about )t/S and7t/ 1 2, respectively, the most intense second harmonic and the thirdharmonic are markedly reduced in their radiation.

It is evident that similarly a number of harmonics can be choked byproviding a number of metal cups at the effective depths of one fourthof desired individual harmonics. Also, the output portion may beprovided laterally to the anode as described above. Still also, theinner face of the insulating body 18 may be covered with a metal film 22as shown in FIG. 4 similarly to the embodiment of FIG. 2.

In the embodiment of FIG. 5 the metal cup 23 is formed of a metallizedcylindrical dielectric body 24 made of, for example, an alumina ceramichaving a central through-hole through which the output conductor 21passes. According to the embodiment of FIG. 5 the positioning of themetal cup 23 is accurate and never tilts due to the passing of theoutput conductor 21 through the central hole of the dielectric body 24.In order to ensure the fixing of the metal cup 23 formed of a metal filmprepared by metallization to the output conductor 21, the fixing is madeofsoldering through a washer 25. However, when the hole of thedielectric body 24 and the output conductor 21 fit closely to eachother, it is possible to solder them without the washer 25. In such acase that the dielectric body 24 exists in the metal cup 23 as shown inFIG. 5, the effective depth 1 for choking the nth harmonic may be takento be about lt/4n \/'e where e is the dielectric constant of thedielectric body 24.

In the embodiments of FIGS. 3 to 5 the mouth of the metal cup 23 may bedirected towards the metal evacuating tube 19. If it is desired tobetter ensure the block ing of only one harmonics, it is sufficient toplace the metal cup 23 at the length to choke the same harmonic that themetal evacuating tube 19 chokes. Though in the above embodiments themetal evacuating tubc I9 is made of a single body. It may be composed ofa plurality of components.

In the embodiment of FIG. 6, in addition to the feature that theeffective depth 1 of the metal evacuating tube 19 is a quarter of thewavelength km of the nth harmonic contained in the microwave output ofthe magnetron 10, a hollow metallic cylinder 26 is inserted in and fixedto the smaller opening part of the stepped funnel shaped metal tube 17.The metal cylinder 26 fomis an annular space or groove 27 between it andthe metal tube 17. The depth 1 of the annular groove 27 is selected tobe a quarter of the wavelength A/n' of the n'th harmonic (n a n). i.e. IA/4n'. On the upper end of the metal cylinder 26 is mounted inregistered relationship and gas-tightly the insulating body 18.

According to the embodiment of FIG. 6 the nth and n'th harmonics arechoked by the metal evacuating tube 19 and the annular groove 27,respectively, on account of their very high series resistances.Consequently, by selecting the effective depths I and I of the metalevacuating tube 19 and the annular groove 27 to be about M8 and A/l2,for example, respectively, the radiation of the second and thirdharmonic components can be reduced markedly. Similarly to theembodiments of FIGS. 2 and 4 a metal film 22 may be laid on the innerface of the insulating body 18 as shown in FIG. 7, in which case theeffective depth I l of the metal evacuating tube 19 is likewise measuredfrom the lower end 22a of the metal film 22.

In the embodiment of FIG. 8 a long dome shaped insulating body 28 iscoupled to the upper end of the metal cylinder 26 in registeredrelationship and gastightly in place of the insulating body 18 in FIG. 6and covering a metal cup 29. The metal cover 20 in FIG. 6 is eliminated.The metal cup 29 is supported by the output conductor 21 at its centerof the bottom from inside. The effective depth l of the metal cup 29 isa quarter of the wavelength of the nth harmonic.

In the embodiment of FIG. 8 the nth and n'th harmonies are choked by themetal cup 29 and the annular groove 27, respectively, due to their veryhigh series resistances therefor. Consequently, by appropriatelyselecting the values of the effective lengths I and 1 the radiation ofthe unnecessary harmonics can be reduced markedly.

In the embodiment of FIG. 9 a flanged sleeve 30 is utilized to form theannular groove 27 in place of the metal cylinder 26 in FIG. 8. The mouthof the long dome shaped insulating body 28 is coupled to the smalleropening part of the stepped funnel shaped metal tube 17 gas-tightly, andthe flange of the flanged sleeve 30 is fixed to the inner face of theshoulder of the stepped metal tube 17. In the embodiments of FIGS. 6 to9 the metal tube 17 and the metal cylinder 26 or the flanged sleeve 30fonning the annular groove 27 may be made into a single integral unit.

In the embodiments of FIGS. 6 to 9 the output portion is provided in theaxial direction of the anode. However, the output portion may beprovided laterally to the anode as described above. In the embodimentsof FIGS. 8 and 9 the metal cup having no sealing member is fixed to theoutput conductor. However, the metal cup may be eliminated with theadvantage of preventing the deformation of the output conductor due tothe provision of the metal cup when the prevention of the radiation ofonly the most intense second harmonic is intended because the mostintense second harmonic can be choked by the annular groove by makingthe effective depth of the annular groove one eighth of the fundamentalwavelength. Further, in the embodiments of FIGS. 6 to 9, if it isdesired to better ensure the blocking of only one harmonic, the lengthsl, and 1 may be determined to choke the same harmonic, i-.e., to beequal with each other, as described with respect to the embodiments ofFIGS. 3 to 5. Furthermore, the mouth of the annular groove is notnecessarily directed towards the vanes.

What is claimed is: a

l. A magnetron comprising a cylindrical anode, a cathode disposedcoaxially with the cylindrical anode, a plurality of vanes protrudingfrom the inner face of the anode towards the cathode for forming cavityresonators, a cathode stem provided gas-tightly at one opening end ofthe cylindrical anode for supporting the cathode and insulating thecathode from the anode, a hollow insulating body having at least oneopening provided on the side of the other opening end of the cylindricalanode in a gas-tight relation with the cylindrical anode, an outputconductor one end of which is con-. nected to one of the vanes and theother end of which extends in the hollow of the hollow insulating body,an output portion composed of at least the hollow insulating body andthe output conductor, and a choke structure provided inside the outputportion for blocking at least one of the harmonics contained in themicrowave output of the magnetron.

2. A magnetron according to claim l, in which the magnetron furthercomprises a hollow metal member having openings one of which isconnected gas-tightly to said other opening end of the cylindrical anodeand a cup shaped metal member having an opening to the bottom of whichsaid output conductor is connected, said hollow insulating body is aninsulating cylinder having two openings, one of the openings beingcoupled gas tightly to the other opening of the hollow metal member, theother of the openings being coupled gas-tightly to the opening of thecup shaped metal member, thereby forming said output portion, and thecup shaped metal member hasan effective depth of a quarter of thewavelength of a particular harmonic of the microwave output and forms achoke structure member for the particular harmonic.

3. A magnetron according to claim 2, in which a cylindrical metal filmis overlaid on at least a part of the inner face of the cylindricalinsulating body in such a manner that one end of the cylindrical metalfilm is electroconductively connected to the cup shaped metal member andthe effective depth of the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.

4. A magnetron according to claim 2, further comprising at least onemetal cup held by the output conductor in such a manner that it ispenetrated by the output conductor at the center of its bottom, themetal cup having an effective depth of a quarter of the wavelength ofone of the harmonics including the particular harmonic and forminganother choke structure member for said one of the harmonics.

5. A magnetron according to claim 4, in which a cylindrical metal filmis overlaid on at least a part of the inner face of the cylindricalinsulating body in such a manner that one end of the cylindrical metalfilm is electroconductively connected to the cup shaped metal member andthe effective depthof the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.

6. A magnetron according to claim 2, in which at least one dielectriccylinder having a central throughhole through which the output conductorpasses and metallized at its side and upper faces is held by the outputconductor, the dielectric cylinder having a height of A V? times thewavelength of one of the harmonics including the particular harmonic andforming another choke structure member for said one of harmonics, wheree is the dielectric constant of the dielectric cylinder.

7. A magnetron according to claim 6, in which a cylindrical metal filmis overlaid on atleast a part of the inner face of the cylindricalinsulating body in such a manner that one end of the cylindrical metalfilm is electroconductively connected to the cup shaped metal member andthe effective depth of the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.

8. A magnetron according to claim I, further comprising a first metalmember connecting gas-tightly the hollow insulating body to the anodeand having a through-hole through which the output conductor passes, theinner face of the first metal member being provided with an annulargroove encircling the output conductor, the annular groove having aneffective depth of a quarter of the wavelength of a particular harmonicof the microwave output and forming a choke structure member for theparticular harmonic.

the harmonics including the particular harmonic and forms another chokestructure member for said one of the harmonics.

10. A magnetron according to claim 9, in which a cylindrical metal filmis overlaid on at least a part of the inner face of the cylindricalinsulating body in such a manner that one end of the cylindrical metalfilm is electroconductively connected to the cup shaped metal member andthe effective depth of the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.

11. A magnetron according to claim 8, in which the hollow insulatingbody is formed into a long dome shape.

12. A magnetron according to claim 11, further comprising a metal cupfixedly supported by the other end of the output conductor at the centerof its bottom and having an effective depth of a quarter of thewavelength of one of the harmonics including the particular harmonic toform another choke structure member for said one of the harmonics.

13. A magnetron according to claim 8, in which said first metal memberconsists of a stepped funnel shaped metal tube with the opening of thelarger diameter part being connected to the anode and a hollow metalcylinder having a diameter substantially equal to that of the opening ofthe hollow insulating body and connected thereto gas-tightly andinserted in and fixed to the smaller diameter part of the stepped funnelshaped metal tube so that the hollow metal cylinder extends to withinthe larger diameter part of the stepped funnel shaped metal tube,thereby forming said annular groove between the hollow metal cylinderand the larger diameter part of the stepped funnel shaped metal tube.

14. A magnetron according to claim 13, in which the hollow insulatingbody is formed into a long dome shape.

15. A magnetron according to claim 14, further comprising a metal cupfixedly supported at the center of its bottom by said other end of theoutput conductor. the metal cup having an effective depth of a quarterof the wavelength of one of the harmonics including the particularharmonic of the microwave output and forming another choke structuremember for said one of harmonics.

16. A magnetron according to claim 8, in which said first metal memberconsists of a stepped funnel shaped metal tube with the opening of thelarger diameter part being connected to the anode and a flanged sleevethrough which the output conductor passes having a flange portion fixedto the inner face of the shoulder of the stepped funnel shaped metaltube, thereby forming said annular groove between the sleeve portion ofthe flanged sleeve and the larger diameter part of the stepped funnelshaped metal tube.

17. A magnetron according to claim 16, in which the hollow insulatingbody is formed into a long dome shape.

18. A magnetron according to claim 17, further comprising a metal cupfixedly supported at the center of its bottom by said other end of theoutput conductor, the metal cup having an effective depth of a quarterof the wavelength of one of the harmonics including the particularharmonic of the microwave output and forming another choke structuremember for said one of harmonics.

1. A magnetron comprising a cylindrical anode, a cathode disposedcoaxially with the cylindrical anode, a plurality of vanes protrudingfrom the inner face of the anode towards the cathode for forming cavityresonators, a cathode stem provided gas-tightly at one opening end ofthe cylindrical anode for supporting the cathode and insulating thecathode from the anode, a hollow insulating body having at least oneopening provided on the side of the other opening end of the cylindricalanode in a gas-tight relation with the cylindrical anode, an outputconductor one end of which is connected to one of the vanes and theother end of which extends in the hollow of the hollow insulating body,an output portion composed of at least the hollow insulating body andthe output conductor, and a choke structure provided inside the outputportion for blocking at least one of the harmonics contained in themicrowave output of the magnetron.
 2. A magnetron according to claim 1,in which the magnetron further comprises a hollow metal member havingopenings one of which is connected gas-tightly to said other opening endof the cylindrical anode and a cup shaped metal member having an openingto the bottom of which said output conductor is connected, said hollowinsulating body is an insulating cylinder having two openings, one ofthe openings being coupled gas tightly to the other opening of thehollow metal member, the other of the openings being coupled gas-tightlyto the opening of the cup shaped metal member, thereby forming saidoutput portion, and the cup shaped metal member has an effective depthof a quarter of the wavelength of a particular harmonic of the microwaveoutput and forms a choke structure member for the particular harmonic.3. A magnetron according to claim 2, in which a cylindrical metal filmis overlaid on at least a part of the inner face of the cylindricalinsulating body in such a manner that one end of the cylindrical metalfilm is electroconductively connected to the cup shaped metal member andthe effective depth of the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.
 4. Amagnetron according to claim 2, further comprising at least one metalcup held by the output conductor in such a manner that it is penetratedby the output conductor at the center of its bottom, the metal cuphaving an effective depth of a quarter of the wavelength of one of theharmonics including the particular harmonic and forming another chokestructure member for said one of the harmonics.
 5. A magnetron accordingto claim 4, in which a cylindrical metal film is overlaid on at least apart of the inner face of the cylindrical insulating body in such amanner that one end of the cylindrical metal film is electroconductivelyconnected to the cup shaped metal member and the effective depth of thecup shaped metal member is measured from the bottom thereof to the otherend of the cylindrical metal film.
 6. A magnetron according to claim 2,in which at least one dielectric cylinder having a central through-holethrough which the output conductor passes and metallized at its side andupper faces is held by the output conductor, the dielectric cylinderhaving a height of 1/4 Square Root epsilon times the wavelength of oneof the harmonics including the particular harmonic and forming anotherchoke structure member for said oNe of harmonics, where epsilon is thedielectric constant of the dielectric cylinder.
 7. A magnetron accordingto claim 6, in which a cylindrical metal film is overlaid on at least apart of the inner face of the cylindrical insulating body in such amanner that one end of the cylindrical metal film is electroconductivelyconnected to the cup shaped metal member and the effective depth of thecup shaped metal member is measured from the bottom thereof to the otherend of the cylindrical metal film.
 8. A magnetron according to claim 1,further comprising a first metal member connecting gas-tightly thehollow insulating body to the anode and having a through-hole throughwhich the output conductor passes, the inner face of the first metalmember being provided with an annular groove encircling the outputconductor, the annular groove having an effective depth of a quarter ofthe wavelength of a particular harmonic of the microwave output andforming a choke structure member for the particular harmonic.
 9. Amagnetron according to claim 8, in which the magnetron further comprisesa cup shaped second metal member having an opening to the bottom ofwhich said output conductor is connected, said hollow insulating body isan insulating cylinder having two openings, one of the openings beingcoupled gas-tightly to the first metal member, the other of the openingsbeing coupled gas-tightly to the opening of the cup shaped metal member,thereby forming said output portion, and the cup shaped metal member hasan effective depth of a quarter of the wavelength of one of theharmonics including the particular harmonic and forms another chokestructure member for said one of the harmonics.
 10. A magnetronaccording to claim 9, in which a cylindrical metal film is overlaid onat least a part of the inner face of the cylindrical insulating body insuch a manner that one end of the cylindrical metal film iselectroconductively connected to the cup shaped metal member and theeffective depth of the cup shaped metal member is measured from thebottom thereof to the other end of the cylindrical metal film.
 11. Amagnetron according to claim 8, in which the hollow insulating body isformed into a long dome shape.
 12. A magnetron according to claim 11,further comprising a metal cup fixedly supported by the other end of theoutput conductor at the center of its bottom and having an effectivedepth of a quarter of the wavelength of one of the harmonics includingthe particular harmonic to form another choke structure member for saidone of the harmonics.
 13. A magnetron according to claim 8, in whichsaid first metal member consists of a stepped funnel shaped metal tubewith the opening of the larger diameter part being connected to theanode and a hollow metal cylinder having a diameter substantially equalto that of the opening of the hollow insulating body and connectedthereto gas-tightly and inserted in and fixed to the smaller diameterpart of the stepped funnel shaped metal tube so that the hollow metalcylinder extends to within the larger diameter part of the steppedfunnel shaped metal tube, thereby forming said annular groove betweenthe hollow metal cylinder and the larger diameter part of the steppedfunnel shaped metal tube.
 14. A magnetron according to claim 13, inwhich the hollow insulating body is formed into a long dome shape.
 15. Amagnetron according to claim 14, further comprising a metal cup fixedlysupported at the center of its bottom by said other end of the outputconductor, the metal cup having an effective depth of a quarter of thewavelength of one of the harmonics including the particular harmonic ofthe microwave output and forming another choke structure member for saidone of harmonics.
 16. A magnetron according to claim 8, in which saidfirst metal member consists of a stepped funnel shaped metal tube withthe opening of the larger diameter part being connected to the anode anda flanged sleeve through which the output conductor passes having aflange portion fixed to the inner face of the shoulder of the steppedfunnel shaped metal tube, thereby forming said annular groove betweenthe sleeve portion of the flanged sleeve and the larger diameter part ofthe stepped funnel shaped metal tube.
 17. A magnetron according to claim16, in which the hollow insulating body is formed into a long domeshape.
 18. A magnetron according to claim 17, further comprising a metalcup fixedly supported at the center of its bottom by said other end ofthe output conductor, the metal cup having an effective depth of aquarter of the wavelength of one of the harmonics including theparticular harmonic of the microwave output and forming another chokestructure member for said one of harmonics.